1. Field of the Invention
The invention relates to a rotational angle sensor, a motor that has the rotational angle sensor, a rotational angle detector that has the rotational angle sensor, and an electric power steering system.
2. Description of the Related Art
When a motor is required to rotate smoothly and highly quietly, for example, when a motor is used in an electric power steering system (EPS), the operation of the motor is usually controlled by supplying sinusoidal currents, of which the phases are offset from each other by an electrical angle of 120°, to a U-phase coil, a V-phase coil and a W-phase coil. Therefore, a coil-type resolver that detects a rotational angle with a high degree of accuracy is conventionally used as a motor resolver that serves as a motor rotational angle sensor, as described in, for example, Japanese Patent Application Publication 11-160099 (JP-A-11-160099).
However, it is difficult to downsize a coil-type resolver because the coil-type resolver has coils. As a result, the cost of producing the coil-type resolver is high. In recent years, magnetic rotational angle sensors that detect a rotational angle with a high degree of accuracy and that may replace coil-type resolvers have been suggested.
For example, Japanese Patent Application Publication No. 2003-75108 (JP-A-2003-75108) describes a method for increasing the detection accuracy. According to JP-A-2003-75108, sensor devices are formed using hall elements as sensor elements, the sensor devices are arranged at multiple positions along the circumferential direction of a magnet rotor, and a correction is made using multiple sensor signals output from the sensor devices.
Japanese Patent No. 4273363 describes a rotational angle detector. According to Japanese Patent No. 4273363, full-bridge circuits are formed. Each of the full-bridge circuit includes a pair of spin valve magnetic resistances that are connected in series in such a manner that the magnetization directions of spin fixed layers are opposite to each other. In addition, a sensor device that outputs two-phase sensor signals of which the phases are offset from each other by an electrical angle of 90° based on a change in magnetic flux caused by the rotation of a magnet rotor is formed by providing the two full-bridge circuits. In this example, a full-bridge circuit that uses giant magnetic resistances (GMR) as spin valve magnetic resistances is formed. The two sensor devices thus configured are arranged at positions that are apart from each other by an electrical angle of 90×n° (n is an integral number) in the circumferential direction of the magnet rotor.
If this configuration is employed, the phase of each of the two sensor signals that are different in phase and that are output from one of the sensor devices overlaps with the phase of one of the two sensor signals that are output from the other sensor device. Accordingly, it is possible to remove waveform distortion by averaging the sensor signals of which the phases overlap each other. Then, the rotational angle is detected based on the two-phase resultant signals derived through the averaging process. As a result, it is possible to drastically increase the detection accuracy.
However, even if the above-described configuration is employed, there still remains a problem to be solved. When a magnetic rotational angle sensor is used in a motor resolver, the number of magnetic poles of a magnet rotor is changed depending on the intended use. More specifically, when a high torque is required, a large number of magnetic poles are formed, whereas when a high speed rotation is required, a relatively small number of magnetic poles are formed. The two sensor devices are apart from each other by an electrical angle of n×90°. The positional relationship between the two sensor devices may vary depending on the number of magnetic poles. Therefore, in the existing configuration, the arrangement of the sensor devices needs to be changed when the number of magnetic poles is changed. Accordingly, there is still room for improvement in this regard.